The wife and brother of Border Patrol Agent Nicholas Ivie, Christy (center) and Joel, hug at his funeral in Utah last month. Nicholas Ivie was killed in what the FBI has called friendly fire.

Photo: George Frey, Getty Images

The wife and brother of Border Patrol Agent Nicholas Ivie, Christy...

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A helicopter believed to be carrying Homeland Security Secretary Janet Napolitano takes off from the Brian A. Terry Border Patrol Station outside in Bisbee, Ariz. on Friday, Oct. 5, 2012. A preliminary investigation has found friendly fire likely was to blame in the shootings of two border agents along the Arizona-Mexico border, the FBI said Friday. (AP Photo/The Arizona Republic, David Wallace) MARICOPA COUNTY OUT; MAGS OUT; NO SALES

A new report into a shooting that killed a U.S. Border Patrol agent says three agents responding to an alarm were apparently in radio contact as they approached from opposite directions before opening fire on each other in the Arizona desert.

A sheriff's report released Friday says the agents were on patrol separately when the call came in about 1:30 a.m. Oct. 2 that an underground sensor aimed at detecting smugglers and illegal immigrants had been tripped.

Agent Nicholas Ivie, 30, approached on foot from the north. The two other agents walked in from the south when Ivie apparently opened fire, eliciting a deadly barrage of return fire from his colleagues.

Ivie was killed. Another agent was wounded. The third wasn't injured.

Questions had swirled as to whether the agents were in radio contact with each other in the rugged, hilly terrain where signals can be spotty. A communication breakdown could have led to the confusion and ensuing shootout.

However, according to the preliminary report by the Cochise County Sheriff's Office, which is investigating the case along with the FBI, the uninjured agent later told authorities "they were in radio communication with Agent Ivie."

"At one point she observed (Ivie) signaling them with his flashlight," according to the report.

The agent, whose name hasn't been released, told investigators "as they were walking up the trail she heard yelling and then observed muzzle flashes in front of her and heard gunfire."

"She drew her weapon and took cover," the report states.

The agent also said she thought she saw several people moving through the darkness and whispering after the shooting.

The FBI declined to comment on the report, citing its own ongoing investigation.

Authorities have said the agents were about 20 yards from each other when the gunfire erupted. Acting Cochise County Sheriff Rod Rothrock has characterized the shooting as accidental, adding he didn't expect any criminal charges to be filed.

The FBI has called it a case of friendly fire but has declined to comment on any other aspects of the investigation.

While the report sheds some light on the shooting, key details remained unanswered, such as just why Ivie opened fire contrary to training that dictates agents should only discharge their weapons once they have identified a suspect as hostile.

"None of them should have started firing unless they were returning fire from a smuggler or whatever the perpetrator might have been," said retired Border Patrol Agent Jim Dorcy.